Null Infinity as a Weakly Isolated Horizon

Abstract

Null infinity (Scri) arises as a boundary of the Penrose conformal completion of an asymptotically flat physical space-time. We first note that Scri is a weakly isolated horizon (WIH), and then show that its familiar properties can be derived from the general WIH framework. This seems quite surprising because physics associated with black hole (and cosmological) WIHs is very different from that extracted at Scri. We show that these differences can be directly traced back to the fact that Scri is a WIH in the conformal completion rather than the physical space-time. In particular, the BMS group at Scri stems from the symmetry group of WIHs. We also introduce a unified procedure to arrive at fluxes and charges associated with the BMS symmetries at Scri and those associated with black hole (and cosmological) horizons. This procedure differs from those commonly used in the literature and its novel elements seem interesting in their own right. The fact that is there is a single mathematical framework underlying black hole (and cosmological) horizons and Scri paves the way to explore the relation between horizon dynamics in the strong field region and waveforms at infinity. It should also be useful in the analysis of black hole evaporation in quantum gravity.